This invention relates generally to filtration systems and more particularly, it relates to a new and improved apparatus for wetting, flushing and performing integrity checks on encapsulated PTFE filters.
As is generally know, microporous membrane cartridge filters such as the Fluorogard ATX-type manufactured and sold by Millipore Corporation of Bedford, Mass., are designed for high flow and high velocity filtration applications and are in widespread use in commercial chemical processing. One of the common uses of these cartridge filters is in wet process cleaning and chemical distribution applications in semiconductor wafer fabrication facilities. These cartridge filters are constructed with a hydrophobic PTFE (polytetrafluroethylene) membrane in a pleated or disk configuration and with PFA (perfluroalkoxy) core, cage and end caps. They function to efficiently remove particles from a broad range of acids, bases, solvents and other aggressive chemicals and thus have compatibility in a wide range of microelectronic process chemical applications. These cartridge filters are typically adapted for use in conjunction with a permanent filter housing so as to serve as a replacement for an existing one therein.
On the other hand, there are also known in the art of disposable-type filters which are formed of a combined PFA housing shell and an encapsulated PTFE filter. These disposable-type filters serve to eliminate the need for handling of hazardous chemicals and minimizes the installation time. These filters are also produced by Millipore Corporation and are referred to as their Chem-Line 1 and Chem-Line 11 filters.
Prior to using either the cartridge filters or disposable-type in filters the chemical processing equipment in the wafer fabrication facilities, it is necessary to pre-wet the membrane of the filters by flowing through or soaking the filter with a typically 60% isopropyl alcohol (IPA) solution in water and subsequently flushing the wetting solution by water. The use of the IPA solution in water is recommended in order to minimize the flammability of the wetting solution. The water flushing serves to remove the IPA solution so as to eliminate the undesirable reaction or contamination of many chemicals with alcohol.
In addition, prior to installation of the filter into the chemical processing equipment, the filter integrity generally needs to be verified. As used herein, the term "integrity" is defined as a condition of being defect free. A number of known methods for non-destructive testing for filter integrity have been employed. Three of these tests are referred to as follows: (1) bubble point test, (2) forward flow test, and (3) pressure hold test. Typically, the testing for filter integrity is performed after the pre-wetting procedure (i.e., after wetting the membrane with the IPA solution) and before the water flushing step.
Perhaps the most frequently used of the filter integrity tests is the "bubble point test." Since the filter membrane is comprised of many small pores, when it is wetted with a liquid, gas at below a certain pressure will not push liquid out of the pores. However, as the pressure is increased above this certain pressure the liquid is forced out of the largest pores of the filter and thus allows the gas to flow therethrough. This certain pressure is referred to as "the bubble point." If this condition occurs below a predetermined minimum pressure value, the filter will be considered to be defective. Nevertheless, this "bubble point test" is not considered to be the most accurate test procedure for large-area cartridge filters. Therefore, "the forward flow test" and the "pressure hold test" are generally the tests which are used. In "the pressure hold test" referred also to as "pressure decay test," the gas pressure is raised to a pre-specified level after the filter has been pre-wetted. If the rate of the pressure decay exceeds certain limits, the filter may be considered defective.
The prior art device for wetting and flushing a cartridge filter is illustrated in FIG. 1 and has been labeled "Prior Art." The device 10 includes a filter housing 12 which is used to contain a filter cartridge 14 to be wetted and flushed. The filter housing is provided with an inlet pipe 16 connected to its one side and an outlet standpipe 18 connected to its outlet connection. The top of the filter housing is provided with a vent valve 20 which is connected with a vent pipe 22.
A solvent solution such as isopropyl alcohol (IPA) is fed into a solution inlet pipe 24 and through to an IPA valve 26. The solution valve is opened and closed so as to allow the solvent solution to fill the interior of the filter housing. A pressure gauge 28 is connected downstream of the valve 26 and to the inlet pipe 16 so as to permit monitoring of the pressure in the inlet pipe 16. Further, ultra-purified water (UPW) is fed to a UPW inlet pipe 30 which is connected to one side of a UPW valve 32. The other side of the valve 32 is connected to a communicating pipe 34 via a flow meter 36. The communicating pipe 34 is connected upstream of the IPA valve 26 and also to the inlet pipe 16.
In operation, the UPW valve 32 and the IPA valve 26 are both initially closed. The filter cartridge that is to be wetted and flushed is inserted into the filter housing 12. Next, the vent valve 20 is opened. Then, the IPA valve 26 is opened and the IPA solution is introduced into the solution inlet pipe 24 until the interior of the filter housing 12 is completely filled. When the IPA solution reaches the top of the vent pipe 22, the filter housing 12 is tipped slightly so as to insure that all of the air has been removed therefrom. The vent valve 20 is then closed. More IPA solution is added to the solution inlet pipe 24 until the level of the IPA solution has reached inside of the filter core and flows out the top of the standpipe 18. The filter cartridge 14 is allowed to soak in the IPA solution for the amount of time recommended by the manufacturer (i.e., approximately 30 minutes).
After the filter cartridge 14 has been pre-wetted, it must be flushed with the UPW before installation in the chemical processing equipment. Prior to flushing, the standpipe 18 is turned sideways and the vent valve 20 is opened, the IPA solution is allowed to drain from the filter housing 12. The vent valve 20 remains open. This flushing step is first accomplished by opening the UPW inlet valve 32 so as to permit the delivery of the UPW at the rate of approximately 2 GPM into the filter housing 12 with the IPA valve 26 opened and the vent valve 20 open to flush all the IPA solution from these two valves. Next, the vent valve 20 and the IPA valve 26 are closed and the UPW is continued to flow into and through the filter housing 12 in order to insure that the IPA solution has been purged. Finally, the UPW is drained from the housing 12 and the filter cartridge 14 is removed.
This prior art device as just described has the disadvantages of being incapable of recirculating the UPW or to remove the same UPW from the device when it is not in use. Since the filter cartridge 14 engages the bottom interior surface of the filter housing 12, there is also created the problem of trapping air in the downstream side of the cartridge. Further, this prior art device suffers from the fact that draining of the filter housing 12 is inefficient and time-consuming. In addition, this device is not able to accommodate encapsulated disposable filters and makes no provision for performing of the integrity test on the filter cartridge.